Final answer:
The requirement for a spreader bar when lifting trusses is dependent on factors such as the span, weight, building codes, and professional recommendations; consulting with a structural engineer or referring to local codes is necessary to determine the correct span threshold.
Step-by-step explanation:
The question about spreader bars and their required use when lifting trusses with a span that exceeds a certain measurement relates to practical knowledge in the field of engineering, specifically within topics such as construction and structural engineering. Unfortunately, without a specific reference or standard in the question, it's not possible to provide a definitive answer. The use of spreader bars is dependent on a variety of factors including but not limited to the span of the truss, the weight of the truss, local building codes, and manufacturer or structural engineer's recommendations.
It is crucial to consult the relevant standards, building codes, or a structural engineer to determine at what span a spreader bar should be used. The use of spreader bars ensures that loads are evenly distributed during the lifting process, which is critical for maintaining the integrity of the trusses and safety on the construction site.